Swimming pools and other pools such as those used for raising Koi or for decoration can be very dangerous for children, dogs, adults who are disabled or under the influence of alcohol, etc. Most home or apartment pools do not have a lifeguard or anyone else in attendance at all times. A person or animal who falls into such a pool and cannot get out is in serious danger of drowning.
Because of this danger, pools are generally required to be fenced or have a sturdy cover. However, these protective devices can be bypassed or the pool can be inadvertently left unprotected.
A number of different alarms have been developed to sense an object falling into a pool and sound a loud, audible alarm at the pool site and/or in selected nearby buildings, such as a house, apartment clubhouse or manager's office, etc.
One type of pool alarm, exemplified by U.S. Pat. No. 4,775,854, to Cottrell includes a floating sensor and a contact means that closes an alarm circuit when the device is tilted by the wave created by an object falling into the pool. Unfortunately, this type of alarm can be easily triggered by wind generated waves, by the floating sensor bumping against a pool side, etc.
Another type of pool alarm, as described in U.S. Pat. No. 3,778,803 uses a submerged or surface hydrophone to detect sound created by an object falling into the pool. However, this type of alarm can be triggered by nearby loud noises, such as loud music, motorcycles, low flying aircraft, in particular helicopters, etc. Heavy rain can generate sufficient noise to trigger such an alarm system.
Other pool alarms of the sort described in U.S. Pat. No. 2,935,582 respond to pressure changes in the water. However, these often have a narrow sensitivity band between sufficient sensitivity to detect a small child or pet struggling in the water while not generating false signals in response to wind generated waves. Water displaced by a child or pet may be so small that the alarm will not be actuated. If the gain is set too high, the detector will pick up pressure changes created by surface waves generated by a strong wind.
Ultrasonic sound waves generated below the water surface have been used to detect an object falling into the water or struggling in the water by the changes in the waves received at a an ultrasonic sound receiver, as described by Dunegan et al. in U.S. Pat. No. 4,747,085. While ultrasonic detection systems are very effective in detecting intruders in rooms or buildings where nothing should be moving, these systems are less effective in pools, where the surface is constantly moving in response to wind and the surface waves caused by the wind can trigger the alarm. Also, these systems are expensive and draw considerable power so are not practical for a battery powered system.
Many of these prior alarm systems are mechanically and/or electrically complex and require complex mounting systems, so that random failures and wearing out may occur making the alarm inoperative without those depending on the alarm being aware of the failure. Excessive false alarms will lead to the system being turned off much of the time, often at the most dangerous times such as at night.
Thus, there is a continuing need for a simple, easily mounted, reliable pool alarm that can detect a small object such as toddler or pet falling into the pool and will not produce false alarms except under the most unusual conditions.